Inductively coupled band-pass filter



Dec. 28, 1948. A 2,457,774

INDUCTIVELY CQUPLED BAND PASS FILTER,

Filed 001;. so, 1944 R.F. AMPLIFIER A I A .AND 1 INT. FREQGONVERTER 1 s z: 23 24/ 21 as 'INVENTOR AP MADISON GAWEIN.

1m. FREQ. AMP. DETECTOR AUDIO maid L I Patented Dec. 28, 1948 2,451,174 mnccrivs r oonrmn Benn-Pass nitrite Madison C'awein, Fort Wayne, Ind., assignor, by

niesnc assignments, to Farnsworth Research Corporationc a corporation ofl Indiana Application October 30, 1944; Serial No. $61 ,063

This invention relates to band pass filters and particularly to a composite inductively coupled filter arranged for passing. a broad band with substantially uniform response.

For modulated carrier receivers in the radio and television field band pass filters are required which are able to pass a wide band of frequencies with substantially uniform gain, While having a sharp cutoif outsideof the; desired pass band. Numerous filters have been designed forthis purpose. However, unless .afairly-complex network. is utilized requiring. a. comparatively large numberof filter elements a wide band-filterv usually shows less amplification in the middle of the band than at the edges thereof.- This nortuniform response of. the band pass filter can be compensated by providing twoor more: amplifier stages coupled by suitable coupling networks, each stage and its associated-networklbeing care-- fully designed to provide for auniformgoveral'lr However, in this case at least two response. amplifier stages arerequi'red forprov-iding a uni form gain of the wide pass band: to be transmitted.

The band pass filter of the invention has particularly utility as a broad-band filter having a substantially uniform fiat response sired pass band. It in connection with intermedi ate constant frequency amplifiers.

It is an object of the present invention, there frequency or" fore, to provide a composite band passfilter ar-y ranged for obtaining a broad-band flat response.

A further object an optimum-coupled band passfilterwhere broad-band resonant impedances are cver-cou-- plecl to obtain a broader pass band with a substantially uniform response without requiring electronic coupling or other compensatingmeansl Another object of the inventionis to provide an inductively coupled band passfilter having a selectivity outside of the band passfilter equivalent to that of four resonant circuits.

may be used,- for instance; 7

8 Claims, (01. time) Preferably, thetwocommon inductance elements are over-coupled with a mutual inductive coupling to provide for a broad-band substantially flat response.

For a: better understanding of the invention,

together with" other and further objects thereof,

reference is made to the following description, taken in" connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing; Fig; 1 is a cir cuit diagram of a composite band pass filter embodying the present invention and utilized as an intermediate frequency or constant frequency filter in a radio receiver, while Fig. 2 is a set of curves illustrating tharelative response plotted against the relative frequency of the filter of the invention.

Referring now more particularly to Fig. 1 of the drawing, there is shown a composite band pass filter in accordance with the invention utilized in a modulated carrier radio receiver. The receiver' il-histrated' is of the superheter'odyne type and includes antenna lflconnected to ground through coil 2. Coil 3; inductively coupled to coil 2, is connected to a radio frequency ampliof this invention" is to provide In accordance with the invention there is' provided a composite band pass filter comprisingtwo identical filter sections. Each of the twofiltersections includes two identical-resonant circuits and, hence, the bandpassfilter. comprises four such resonant circuits. Each, of, the resonant circuits has a capacitance elementand two in ductance elements. ments is common to for providing a self-inductive coupling thera- One or the inductance eletwo of the resonant circuits resonant circuits; I g 7' cludes shunt condenserZO', series coil 2| and shunt between. These two common inductance ele ments are inductively coupled to each other.

fier and intermediatei frequency converter shown diagrammatically at 4. The output of radio frequency amplifier and intermediate frequency con verter disconnected by'lea'd 5 to band pass filter 6. The output of the band pass filter 6' is connectedth'rough lead 7 to intermediate frequency amplifier ,8. Band pass filter 6 is coupled to the intermediate ir'e'quen'cy converter 4 through blocking condenser m. Connected in cascade to intermediatefrequcncy amplifier 8 is second detector l2; audio" amplifier" l3 and loud speaker l4. Theportions'of the receiver described abcwe,v

with the eicceptioriof bandpass filter 6, may all be of conventional well-known construction so that detailed illustrations and descriptions thefeof are deemed to be unnecessary.

Band pass filter 6 embodying the present invention includes two identical filter sections [5, l6: Filter' section li comprises" two identical The first resonant circuit incoil 22.

identical denser 23,

The second resonant circuit which is to the first circuit includes shunt conseries cbil ffa'nd'shuntcoil fi. Hence,

it will be seen that shunt coil 22 is common tothe two resonant circuits just described and provides a self-inductive coupltng therebetween.

Filter section I6 is identical with filter section l5 and also includes two identical resonant cir- 3 cults. The first resonant circuit of filter section l6 includes shunt coil 25, shunt condenser 26 and series coil 21, while the second resonant circuit consists of shunt condenser 28, shunt coil 25 which is common to the two resonant circuits and series coil 30. Filter sections l5 and I6 are mutually inductively coupled through shunt coils 22 and 25. Series coils 2|, 24, 21 and 30 are not inductively coupled to each other as indicated by shield 3| illustrated between series coils 24 and 21; if desired a similar shield may. bear.- ranged between series coils 2| and 24 and between series coils 21 and 30.

The band pass filter of the invention consists of four identical circuits which are all tuned to the of a composite band pass filter of this type depends mainly upon the mutual inductive coupling between coils 22 and 25. The optimum coupling which Will give a substantially uniform. broadband response with a good selectivity outside of the pass band can be calculated in the following manner. We will assume that each of condensers 20, 23, 26 and 28 has the same capacitance value 0. We will also assume that each of series coils 2|, 24, 21 and 30 has the same inductance value L. The inductance of each of coils 22 and 25 is assumed to be identical and has the same resonant frequency. The response curve value M. The mutual inductive coupling between coils 22 and 25 is M.

We define now and Q is the ratio of the reactance of any one of series coils 2|, value may be determined by over the resistance where R is the ohmic resistance of any one of the four series coils of the band pass filter, and f0 is the resonant frequency of any of the four resonant circuits. If a load resistor is connected to any one of the four resonant circuits,its resist- 24, 21 and 30. Its

With this assumption the relative response A of I the band pass filter of the invention can be calculated, and the result is as follows:-

in being the resonant frequency and f a variable 4 frequency. The resonant frequency may be obtained by the following relationship:

The above formula can be rewritten by dividing the nominator and denominator by 2 Hence,

cslp

and

This equation can be solved for at when s is treated as a parameter. In this manner a, family of curves can be obtained, and 9:, the relative frequency can be plotted against the relative response. 11 may be treated as a constant because it depends upon in, the resonant frequency which is constant. For f=fo, :c=O. Furthermore, the formula for the relative response is symmetrical in x; changing the sign of a." will not change the value obtained for A.

It will be appreciated that the above calculations are greatly facilitated because the band pass filter of the invention includes four identical resonant circuits. Besides, the type of coupling between the circuits is particularly simple. Calculation of a quadruply-tuned circuit without the assumption of suitable simplifications presents almost unsurmountable difliculties.

Fig. 2 illustrates the relationship between the relative frequency w and the relative response lowing manner:

. s 11 therefore,

s== 8'=M=M8' P M L By substituting the optimum value for s as explained hereinabove M can be calculated.

In order to design a band pass filter in accordance with the invention the following procedure may be adopted. At first a value for resonant frequency in, and for L the inductance of any one of coils 2|, 24, 21 and 30 should be chosen. Then, the inductance of coils 22 and 25 may be calculated as follows:

gamma capacitance of condensers: 2m. 23"; 28 and.

1 chromatin.

Now, M may be determined from the following relationship any M L. and, therefore, w

2 M Zs which in turn can be expressed as follows:

ML s W The: band passfilten: of. the; invention: hasrlieen; showm as the. filterbetweem intermediate frequency con erten 3 am. intermediate frequency amplifieirlli- Ifzmorerthan one intennedi ate frequency: amplifier stage is provided. the band: pass. filter: of: therinyentionimay be; arranged; betweena every? tww. subsequentstages; Blocking condenser tlllsenvcstrr separate; the-.direct current: from. platecircuit; off the tuba of. mill-d frequency amplifier intermediate: frequency converter t irontthesinnuti-circniit oiiintermediate: frequency amplifier 8. However, it is tabs-under stood that; blocking. condenser l0 forms nopart of band pass filter 6.

It is true that the band pass filter of the invention operates only accurately at a. predetermined frequency. However, itlwyill function reefsonably well within. a. certain frequency range. To this endcondenserslfl; wand 28 may all be made'llariable and gangeldstog ether so thantiw resonant frequency. 10, will be. adiustable within a certain range. The numerical values of the. circuit elements should be calculated in accordance with the above formulas for the mid-frequency of this range. Thus a certain adjustment of the resonant frequency may be provided.

Referring briefly to the operation of the superheterodyne radio receiver illustrated in Fig. 1, modulated carrier signals intercepted by antenna I are coupled by means of coils 2 and 3 to radio frequency amplifier and intermediate frequency converter 4. Here, the intercepted signal is amplified and converted into an intermediate fre-' quency signal. The intermediatefrequency signal derived from converter 4 is coupled to band pass filter 6 through coupling condenser l0. Band pass filter 6 passes the intermediate frequency signal with its side bands to intermediate frequency amplifier 8. The intermediate frequency is amplified in amplifier 8 and connected in turn to detector stage 12. The modulation components of the signal are derived from detector stage I! and are amplified by audio amplifier l3 from which they are supplied in the usual manner to loud speaker l4.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifictitious: as fall" within: the o1! thezmrention.

is claimed: is:

I; composite band; pass; filtercomprising: two:

true spirit and scam idcnticali filter sections; each of said filter. sections;

including; two: identical resonant circuits, each of said resonant; circuits having a capacitance. element: and: two inductance. elements; one. of said inductance elements-being common to two. of said 10' cimntts; fnirpro idinga.selfeinrluctance coupling thenebctween. saidi two. common inductance: elements beings inductively coupled. to each other.

it Acompnsite band pass filter comprising two identical filter sections; each. of said filter sections includl'ng-twoti'dentical' resonant circuits, each of said; resonanticircuits: having a capacitance. elementan'd -two inductance. elements, one of said inductance elements being common to twov of said circuits tor providing a self-inductive. couplinmtherebetween, saidtwo common inductance elements. being; over-coupled: witha mutual. in.- dimtive: coupling thereby to provide for a. broadband substantiallvfiat response, means for supplying an input: signal. acrossv a. capacitance. element ofi onemi said filter. sections, anctmeans: for

' deriving an output signalv across a capacitance element ofithe other one of said filtersections.

3;. A11 composite: band pass filter comprising; two. identical filter-"sections, each. of said filter sections including two? identical resonant circuits, each of said resonant circuits. having a capacitance element and inductance elements; one havingv an: inductance L. and the other one having an inductance said; inductance element. having" am inductance: being common to, two: of said circuits for providing a self-inductive. coupling thcrebetween', said two. common inductance; elemental-laying; the-inductanceM being coupled to each other with; a: mutual. inductive. coupling; M soithaii has avalue substantially between M r and 4. A composite band pass filter comprising two identical filter sections, each of said filter sections including two identica1 resonant circuits, each of said resonant circuits having a capacitance element and two inductance elements one having an inductance L and the other one having an inductance M, said inductance element having and M 2.253 being equalto where Q is the ratio of the reactance over the resistance of any one of said inductive elements having the inductance L, means for'supplying an input signal across a capacitance elementcof one of said filter sections, and means for deriving an output signal across a capacitance element of the otherone of said filter sections.

5. A composite band pass filter comprising twoidentical filter sections, eachvof said filter sections including two identical resonant circuits,-each of said resonant circuits having a capacitance element and an inductance element connected in parallel and another inductance element connected .in series, said series inductance element being common to two of said circuits for providing a self-inductive coupling therebetween, said two common inductance elements being induc tively coupled .to each other,

6. A composite band pass filter comprising two identical filter sections, each of saidfilter sections including two identical resonant circuits, each of said resonant circuits having a capacitance e1ement and an inductance element connectedin parallel and another inductance element connected in series therebetween, said seriesinductanceelement being commonito two of said circuits for providing a self-inductive coupling therebetween, said two common inductance elements being optimum-coupled to each other with. a

mutual inductive coupling therebetweento pro-I vide fora broad-band substantially fiat response, means for supplying an input 1 signal across a capacitance element of one of said filter sections,"

and means for deriving an output signal across a capacitance element of the other one'of said filterv sections.

7. A composite band pass filter'comprising two identical filter sections, each of said'filter sections including two identical resonant circuits," each of said resonant circuits having a capaci-i tance elementv and an inductance element havin an inductance L connected in parallel and, another inductance element having an inductance M connected in series therebetween, said inductance "element: having! theinductance M bein common :-tortwo of said circuits for providing a,

self-inductive coupling therebetween, said two common inductance elements having the inductance M being coupled to each other with a mutual one" of "said filter sections, and means for deriving eachother.

an o'utputsignal across a capacitance element.

of theother one of-said-filter sections. i

8. A compositebandpass'filter comprising two identical filter'sections, each of said filtersections including two identical resonant circuits; every two of saididentical resonant circuits having a common inductance element for providing a self-=' inductive coupling therebetween, said common inductance elements being inductively coupled to tMAnIs ocAWEm. 'REFERENCES ems file of this'patent:* p NITED STATESCPA'IFENTS'H Number 3 Name Date 1,938,620 Braden Dec. 12, 1933 Therenowmg references are of'record in the" 

